target/riscv: Use true diff for gen_pc_plus_diff

target/riscv: Use true diff for gen_pc_plus_diff

Reduce reliance on absolute values by using true pc difference for
gen_pc_plus_diff() to prepare for PC-relative translation.

Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230526072124.298466-6-liweiwei@iscas.ac.cn>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>